1. Field of the Invention
The present invention relates to the field of electrically conductive printable materials, and more particularly to materials which may be used for electrical components requiring curing at high temperatures, for example at 200.degree. C. to 275.degree. C. or higher. Still more specifically, the present invention relates to an electrically conductive composition which includes a thermoplastic resin binder, conductive particles dispersed therein and a solvent. Still more particularly, in connection with the manufacturing techniques, the thermoplastic resin is added in two different stages during the dispersion process to improve the properties of the final product.
2. Description of the Prior Art
Electrically conductive printable compositions have numerous applications. Electrically conductive printable compositions are used in the capacitance humidity sensor which is the subject of co-pending, commonly assigned U.S. patent application Ser. No. 07/831,553 filed in the name of Paul E. Thoma on Feb. 5, 1992. In this device, a polyimide film is mounted to a frame and is screen printed with first and second layers of conductive material which are then fused to the polyimide layer. A conductive ink is then screen printed onto the conductive layers and fused to provide an electrical contact. The conductive ink used in the prior devices of the type described in the aforementioned patent application was Polymer Conductor P-2313 manufactured by EMCA-REMEX Products of Montgomeryville, Pa. A Technical Information Bulletin on this material is included with this application for patent. This material is a silver containing ink having a sheet resistance of 0.060 ohms/square/14 microns cured, a viscosity of 30,000 cps .+-.10,000 cps at 25.degree. C. using a Brookfield HBT Viscometer with 13 R small sample adapter chamber and #27 spindle at 10 rpm. The cure schedule for this material calls for 165.degree. C. for 30 minutes in a circulating air convection oven or belt furnace.
One of the problems with this material is that the low cure temperature is not suitable for some applications. For example, the fusion temperature used in the process for manufacturing a capacitance humidity sensor has typically been 250.degree. C. It has been determined that this cure temperature results in poor adhesion of the capacitor's plates to the dielectric layer beneath the silver ink of the prior description. One way to overcome the problem would be to change the oven temperatures, but changing the oven temperature may result in long holds in the process, and the chance for processing the sensors at an incorrect temperature is also greater when the oven temperature is being changed for different parts of the process.
There are two other relatively undesirable aspects of the P-2313 material. One is its relatively short shelf life of four (4) months when stored at 5.degree. C. and the relatively wide viscosity range, i.e. .+-.10,000 cps with a starting viscosity of 30,000 cps. In other words, the potential range of viscosity is between 20,000-40,000 cps.
It would be desirable to have an electrically conductive composition suitable for screen printing applications which could be fused or cured at higher temperatures. Desirably, the composition would have no chemical reactions involved in its preparation and have a relatively simple manufacturing process, a long shelf life and form a strong adhesive bond between the fused composition and a variety of substrates, including polyimides. Such a composition would represent a significant advance in this technology.